gallivm: (trivial) fix typo bug with small AoS format unpacking
gallivm: (trivial) fix typo bug with small AoS format unpacking

Fix typo using wrong (uninitialized) build context introduced by
4634cb5921b985f04f2daf00cda2d28036143bd3. (This only affects very rare
small packed formats which have a PIPE_SWIZZLE_0 channel, such as
r4a4, which is never used by mesa/st. Nevertheless it broke lp_test_format.)

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler